Percussion boring machines are principally used for laying service lines, especially underground pipelines, without trench-digging, and require run monitoring and control means which allow the machine to be guided accurately to its target. Thus the machine must go round obstructions in the ground. If not, these, like differing soil formations, often lead to directional deviations which need careful correction, since otherwise the target may be missed by a large amount, particularly over long distances. Further, problems arise in the event of unevenness in the ground surface which does not allow the percussion boring machine to be guided at a constant distance from the surface, since the machine would then follow the irregularities of the surface.
In order to avoid the above-mentioned difficulties, it is, for example, known from EP-A 0 617 193 to fit a transmitter in the striking tip or in a displacing head. The transmitter is Located immediately in front of the front wall of the machine housing to which the striking piston which reciprocates in the machine housing imparts its whole impact energy at high frequency. Accordingly the transmitter is subjected to extremely high mechanical stresses, which necessitate costly measures to provide adequate protection of the transmitter from damage.
A further disadvantage of such head-mounted transmitters is that the tip of the machine is the most highly stressed part, and has to be made correspondingly stable and connected securely to the housing. This is particularly true in the case of a striking tip which is guided to be axially movable in the machine housing. The transmitter is therefore only accessible with difficulty, and as a result is difficult to maintain and repair.
Furthermore the tip of the machine serves to penetrate and break through stones and the soil, and it is therefore subjected to severe stresses from the surrounding ground.
An earth boring machine provided with a transmitter for run monitoring is already known from U.S. Pat. No. 4,674,579, but this is not propelled by dynamic impact means, so that less mechanical stress of the transmitter is involved. Nevertheless in this machine the transmitter is not located in the head but in the front part of the rear third of the machine housing. As a result of its length, the transmitter requires, just as in the case of the transmitter located in the head of the percussion boring machine according to EP-A 0 617 193, a not inconsiderable amount of space, which is necessarily associated with a corresponding increase in the length of the machine. However, in the case of percussion boring machines for laying lines, especially pipelines, without trench digging the length of the machine represents a particular problem, since laying the pipes mostly takes place from a starting pit. For such a starting pit there is usually little space available, for example in front gardens, under sidewalks or in existing inspection chambers. As result the machine has to be as short as possible in order to manage with narrow starting pits. The same applies analogously to the target pit.